The purpose of this investigation is to isolate and identify the side chain cleaved metabolite formed in rat intestine from 1,25-dihydroxyvitamin D3 and determine its role in phosphate and calcium homeostasis. SPECIFIC AIMS: 1) The isolation and identification of the intestinal side chain cleaved metabolite formed from 1,25-dihydroxyvitamin D3. 2) The determination of the biological role of this compound in calcium and phosphate homeostasis. 3) The determination of the subcellular location of the enzyme responsible for the side chain cleavage of 1,25-dihydroxyvitamin D3. 4) The study of the factors regulating this process. 5) The determination of whether the 24 hydroxylation of 1,25-dihydroxyvitamin D3 is in any way related to side chain oxidation. 6) The study of the metabolism of the side chain oxidized metabolite in vivo. 7) Synthesis of the side chain cleaved metabolite. PROCEDURE: 1) The generation of appropriately radiolabelled compounds. 2) Procedures for the generation of the metabolite. 3) The purification of the metabolite so that it can be identified chemically. 4) Chemical identification of the metabolite by uv spectroscopy, mass spectroscopy, chemical modification, and nuclear magnetic resonance. 5) Testing the biological activity of the metabolite. SIGNIFICANCE: Preliminary experiments have demonstrated that the metabolite formed in the intestine by the side chain cleavage of 1,25-dihydroxyvitamin D3, is capable of increasing the serum phosphate level in the hypophosphatemic, vitamin D deficient rat. The metabolite may play an important role in the maintenance of the serum phosphorus and skeletal and dental integrity.